1. Field of the Invention
This invention relates to a semiconductor light-emitting device and, more particularly, to a high-brightness semiconductor light-emitting device that has a transparent conductive film as a current spreading layer.
2. Description of the Related Art
In recent years, the crystalline quality of GaN-based or AlGaInP-based semiconductors is enhanced since they can be grown by a MOVPE (metalorganic vapor phase epitaxy) method. Thus, a high-brightness blue, green, orange, yellow, and red light-emitting diode (herein referred to as LED) as a semiconductor light-emitting device can be now manufactured.
However, in order to achieve the high brightness, the current spreading property needs to be improved such that an applied current is uniformly supplied into a chip plane of an LED. For example, an AlGaInP-based LED device is fabricated such that the current spreading layer has a large thickness of about 5 to 10 μm. Therefore, the cost of materials required for the growth of the current spreading layer increases, which causes an increase in the manufacturing cost of the LED device. Thus, the AlGaInP-based LED device is difficult to fabricate at low cost.
In consideration of this, a method is proposed that an ITO (indium tin oxide) or ZnO (zinc oxide) film is used as the current spreading layer to get a sufficient translucency and good current spreading characteristics (JP-A-8-83927). Further, a method is proposed that an ITO film is directly formed on a p-type clad layer (see U.S. Reissued Pat. No. 35665 and U.S. Pat. No. 6,057,562).
When the ITO film is used as the current spreading layer, the conventional method of increasing the thickness of the semiconductor layer as the current spreading layer to about 5 μm to 10 μm is not necessary, and the formation of the epitaxial layer can be saved by that much. Thus, the high-brightness LED device and the epitaxial wafer for the LED device can be manufactured at low cost.
However, when the ITO film is used as a window layer, a contact resistance is generated between the semiconductor layer and the ITO film of a metal oxide, and a forward voltage disadvantageously increases. More specifically, the ITO film used as a transparent conductive film (transparent electrode) is an n-type semiconductor. On the other hand, the upper clad layer contacting the ITO film is a p-type semiconductor. Therefore, when a forward voltage is applied to the LED, a reverse bias is established between the transparent conductive film (transparent electrode) and the p-type clad layer. Because of this, a large voltage (i.e., increased operating voltage) has to be applied to flow current therethrough.
To solve this problem, a method is proposed that a p-type contact layer is formed between the p-type clad layer and the ITO film to offer a tunnel junction which allows the LED to be driven at a low voltage (U.S. Reissue Pat. No. 35665). In order to drive the LED at a low voltage by the tunnel junction, the p-type contact layer is composed of an As-based high-carrier concentration layer that a p-type dopant such as Zn is generally doped at a high density of 1×1019/cm3 or more.
The contact layer needs to be formed of a thin film since it can be a light-absorbing layer to light emitted from the active layer. Further, it needs to have a high carrier concentration to achieve the tunnel junction. Therefore, the dopant diffusion is likely to occur due to heat generated during the growth. Especially, in case of forming the high-carrier concentration contact layer on the p-type clad layer, the distance between the contact layer and active layer becomes short so that the diffusion is increased. As a result, the following problems will occur.
The p-type dopant is diffused from the contact layer to the depth direction of the LED device. When the dopant reaches the active layer of the LED device, the dopant causes a defect in the active layer. The defect will compose a nonradiative recombination component to lower the optical output of the LED device.
Further, since a substantial carrier concentration of the high-carrier concentration contact layer lowers due to the dopant diffusion, the tunnel junction is difficult to obtain and the tunnel voltage is increased. For this reason, the drive voltage (forward voltage) of the LED device disadvantageously increases.
In the above method of forming directly the high-carrier concentration layer on the p-type clad layer and forming the ITO film thereon, the dopant is likely to reach the active layer due to the thin p-type clad layer, whereby the optical output lowers and the reliability degrades.
Further, due to the thin p-type clad layer, the device is frequently broken by damage in wire bonding.
As a solution to solve the problems, it is effective to provide a buffer layer to suppress the diffusion of the p-type dopant, Zn between the high-carrier concentration contact layer and the p-type clad layer. The buffer layer is suitably made of AlGaAs or AlAs. This is because these materials are optically transparent to emission wavelength, their crystals are easy to growth as compared to four-element material such as AlGaInP-based materials, and they are almost lattice-match to the AlGaInP-based material to compose the active layer to lower the operating voltage of the LED device. For example, a method of forming the buffer layer is proposed that an AlGaAs layer with a lower resistivity than the p-type clad layer is formed to increase the distance between the active layer and the contact layer (U.S. Pat. No. 6,057,562).
However, the diffusion may be contrary promoted since the buffer layer is doped with a quantity of the dopant so as to allow the buffer layer to have a lower resistivity than the p-type clad layer. Especially, the diffusion can be pronounced when the buffer layer is composed of a semiconductor material containing As as a group V element and transparent to the emission wavelength, e.g., AlGaAs with a high Al ratio in mixed crystal.
Further, when the p-type clad layer has a high C (carbon)-concentration, the diffusion of the dopant becomes much more pronounced. Therefore, the optical output and the reliability lower or degrade significantly. This problem cannot be so much solved even if the distance between the contact layer and the active layer is increased.
On the other hand, it is found by the inventors that the dopant diffusion is further caused by that the buffer layer or p-type clad layer contacting the contact layer has a high H (hydrogen)-concentration, i.e., the dopant diffusion is pronounced as the H-concentration is increased, thereby causing a reduction in optical output and an increase in operating voltage.